The present invention relates to a protection element responsible for the protection of an ordinary semiconductor element included in a semiconductor device from a voltage higher than the normal voltage which the ordinary semiconductor element is allowed to receive. More specifically, this invention relates to an improvement applicable to a structure of an element which has the purpose of protecting an ordinary semiconductor element included in an integrated circuit from having applied thereto a voltage higher than expected.
The magnitude of an electric voltage allowed to be applied to an integrated circuit is rather limited, because the geometrical dimensions of an integrated circuit are fairly small. It is therefore preferable that each ordinary semiconductor element included in an integrated circuit be associated with a specific element which is responsible for discharging an unexpectedly high voltage to ground, whenever such a high voltage is applied to the ordinary semiconductor element included in the integrated circuit, for the ultimate purpose of protecting the ordinary semiconductor element from being destroyed by such a high voltage. In reality, each integrated circuit contains a group of protection elements having the foregoing function. Since ordinary semiconductor elements of an integrated circuit are designed to work with a relatively low voltage, e.g. 5 V, the working voltage of such protection elements is ordinarily selected to be in the range of 6 to 7 V. However, some discrete elements such as fluorescent type display elements operate in a voltage range of 40 through 50 V and are connected with some of the ordinary semiconductor elements included in an integrated circuit. This requires a protection element which has a working voltage which exceeds the aforementioned voltage range of the protection elements available in the prior art, e.g. 6 to 7 V.
This simplest structure of such a protection element as described above is a p-n junction applied with a reverse bias, which breaks down under the condition that the associated or protected ordinary semiconductor element has applied thereto a voltage higher than the allowable voltage.
An improved structure for such a protection element as described above is a type of parasitic MOS field effect transistor in which the gate and drain are connected to a protected element and the source is grounded. The parasitic MOS field effect transistor is designed to become conductive exclusively under the condition that the drain has applied thereto a voltage identical to or higher than the threshold voltage of the protection element. Therefore, the threshold voltage of the protection element is required to be slightly more than the working voltage of the protected element. Thus, the impurity concentration of the channel stopper region must be relatively high. However, a high impurity concentration has a drawback which results in a lower breakdown voltage between the drain region and the channel stopper region, making it difficult to produce a protection element of the parasitic MOS field effect transistor type having a high working voltage.
Another improved structure for such a protection element as described above is a lateral bipolar transistor in which the emitter is connected to a protected element and in which the base and collector are grounded. The lateral bipolar transistor is designed to become conductive exclusively under the condition that the emitter has applied thereto a voltage identical to or higher than the threshold voltage of the lateral bipolar transistor serving as a protection element. In other words, a p-n junction between the emitter region and the substrate or between the emitter region and the base region which actually is the channel stopper region is broken down when a voltage identical to or higher than the working voltage of the protection element is applied to the emitter. This charges up the substrate. As soon as the potential of the substrate exceeds the potential of the collector, the lateral bipolar transistor becomes conductive and functions as a protection element. Accordingly, the working voltage of the lateral bipolar transistor type protection element is actually determined by the breakdown voltage between the emitter region and the substrate or by the breakdown voltage between the emitter region and the base region which actually is the channel stopper region. However, since the essential object of the channel stopper is to prevent the parasitic transistor function from occurring for the entire area of an integrated circuit, the amount of impurity concentration is rather limited for the channel stopper region, making it difficult to produce a protection element of a lateral bipolar transistor type having a high working voltage. In reality, even if the amount of the impurity dose is selected to be as low as 10.sup.13 /cm.sup.2, it is difficult to make the working voltage of such a protection element higher than 30 V.
Accordingly, any structure of the protection elements available in the prior art is not satisfactory for production of a protection element having a working voltage of 40 V or higher.